Experts say the Christchurch City Council failed to do a vital safety check on its controversial pressurised waste water tanks.
A report requested by the council from independent electrical inspector Alan Cuthbert said because of sewage creating methane gas, and the use of electrics in the new tank system, there was potential for an explosive atmosphere to develop.
Cuthbert said the council had failed to carry out a hazardous area classification on the tanks, which was required by law.
More than 5000 pressurised tanks are scheduled to be installed across the city to replace the earthquake battered gravity system - 1098 have been completed.
Householders in Parklands and North Beach have objected to the new tanks.
If the city council did not work quickly to remedy the issue it could be charged and fined, Cuthbert said.
Worksafe New Zealand Canterbury rebuild programme director Kathryn Heiler said it was investigating the issue to see if there were any breaches of safety laws.
Council water and waste manager Mark Christison said it had taken action to address the issue.
"A hazardous area classification is just a more intense risk assessment and we are looking at that right now, including putting gas meters into selected systems to gauge risk.
"People can be reassured that the city is not going to invest in systems that are dangerous.
"We feel we have done our due diligence with our assessments and will complete the hazardous classification to sign off any final issues in design."
Christison said it was unlikely that the systems would produce methane gas.
"It would take solids sitting in decay under anaerobic conditions, which would take a certain amount of time.
"The systems are emptied several times a day so they never fill up past one third full.
"Over a million of these [systems] have been installed in the United States and even in New Zealand there is not one single report of an explosion."
Garry House, chairman of a joint Australian and New Zealand committee for hazardous areas and explosive atmospheres, said the council was "grossly negligent" in not doing the hazardous area classification before installation.
"It is like Pike River. They hadn't done the same hazardous area assessment.
"But with Pike River people knew that coal was hazardous - but not everyone knows how hazardous sewage is," he said.
House is also chairman of the group protesting against the new tanks, Christchurch Concerned Citizens.
Christison said: "Look, we've been given a lot of so-called issues from this group. We have tried to research them all.
"I understand there is a lot of pressure and stress out there due to land classifications ectera but all the city is trying to do is put in a more sustainable system."
- The Press
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